(a) Field of the Invention
The present invention relates to an organic electroluminescence (EL) display panel and a display thereof, and more in particular, to an organic EL display panel and an organic EL display having the same, which are simple and can overcome the limit of gray display due to characteristic distributions of TFTs.
(b) Description of the Related Art
As displays used currently, there are the most employed CRTs, and the ratio of LCDs for computers is gradually increasing. However, the CRTs are too heavy and have large volume, and the LCDs are not bright and cannot be seen from the side and are low in the aspect of effectiveness, thereby not satisfying users perfectly.
Thus, many researchers try to develop cheaper, more effective, thinner and lighter displays, and one of attractive displays as a next generation display is an OLED (Organic Light Emitting Device).
This OLED uses electroluminescence (EL: a phenomenon of generating light on applying electricity) of specific organic materials or high molecules, and it is not provided with back light. Accordingly, since the OLED has advantages that it can be slim and manufactured simply and at lower cost and has a wide viewing angle and generates bright light compared with the LCD, a research for it is vigorously proceeding worldwide.
FIG. 1 is a circuit diagram to illustrate an example of a normal organic EL driving device.
Referring to FIG. 1, a normal organic EL driving device is composed of a switching transistor Qs, a storage capacitor Cst, a driving transistor QD and a organic EL device OLED.
At the time of operation, the organic EL display employs not a passive driving method that light is emitted only in selecting one transverse scan line, but an active driving method, which enlarges the light-emitting duty considerably, since its brightness is low relative to CRT. In this case, an active layer of the light-emitting cell emits light in proportion to an injected current density.
However, there is a problem that the conventional organic EL driving device is very simple in a driving circuitry thereof but must be applied with voltages from an external device and also the organic EL device must be applied with voltages.
In common, the organic EL device is difficult to display gray since it is very sensitive to applied voltages and its variation is very large.
In order to overcome these disadvantages, a cell driving unit is provided that current are applied from an external device and a cell is applied with current in an amount of the applied current, as shown in FIG. 2 of SID 01 Digest p.384.
FIG. 2 is a circuit diagram to illustrate another example of a conventional or EL device.
Referring to FIG. 2, a brightness data as current is applied from an external device so as to apply the current to the organic EL cell, and selection signals are applied to respective horizontal lines of a screen at corresponding timing to supply the brightness data to a specific coordinate.
This supplied brightness data enables to flow current with the same value as the current to the organic EL device through current mirror circuits QS1 and QS2. Thereafter, when the present horizontal line is stopped and that of the next stage is selected, the storage capacitor Cst keeps a gate voltage of the TFT QS2 required to flow the current to the organic EL device and thus it enables to flow the current to the organic EL device until new brightness data and a horizontal line (column line) selection signal are inputted.
However, the aforementioned FIG. 2 has problems as following.
That is, a specific scan line SCAN 1, and a current mirror operation and a current storage operation selection wire SCAN 2 is needed as a pair.
This has problem that the number of a vertical scan wire and a driving become double to be disadvantageous in terms of production yield and price thereof.
In addition, since the brightness data is a current, an existing voltage-driven IC cannot be used, and a current-driven IC has a difficulty more than the voltage-driven IC, in terms of techniques.
Moreover, the characteristics of a plurality of thin film transistors (“TFTs”) provided in the respective cells of the organic EL must be uniform in order to be operated as current mirrors, and the characteristics among the TFTs provided in the other cells must be uniform, too. The TFTs as current mirrors operate not in a switching mode but in an active mode, and thereby requiring optimal characteristics.
In addition, when the variation of the characteristic in threshold input voltage of the TFT to supply a current to the organic EL occurs, the variation of output occurs to supply currents different from the brightness data, thereby causing the variation of brightness. Thus, it is difficult to implement a detailed gray.
As described above, for all increasing the number of the vertical scan lines and the driving IC, the conventional organic EL driving cell has many problems.